There are a lot of parasitic power drains in modern vehicles. They probably just failed to remove the battery cable if it does not have a master that cuts power to everything.
Yes. New car manufacturers that ship cars usually have a “battery shut-off switch” that gets removed before sale.
I discovered this myself the hard way although by accident. I left my car keys in the ignition, Everything was off no lights, heater, radio, or anything else left on.
I go to start the car a few days later... oh there is my other set of keys.... The battery was totaly dead.... had to get a jump start.